The present invention relates to a method and apparatus for attenuating the sound emanating from a bank of a plurality of vibratory screening machines which are operating simultaneously.
By way of background, vibratory screening machines are used for the purpose of separating particulate materials. Each machine usually has a plurality of vibrating screens mounted thereon. The material to be separated is passed along the vibrating screens, and the finer material passes through the screens whereas the coarser material runs off of the end of the screens. The screens generally vibrate at a frequency within a range of between about 14–60 Hz. If the screens are of a very fine mesh or while they have material covering them in the course of normal operation, they can essentially function as loud speakers. Due to slight variations in the speed of the motors which vibrate the screens, the frequency of the individual machines will vary from one to the other by a few percent. This gives rise to a changing phase relationship among the various machines which causes a cyclic “droning” where the sound becomes louder and softer as the machines go in and out of phase with each other. This cyclic droning, which occurs generally within a typical period of between about 10–20 seconds, is particularly annoying to operating personnel or nearby residents.